In the incineration plants such as incineration plants for municipal solid waste, industrial waste and/or medical waste, the dioxides precursors including organic compounds such as phenols, benzene and acetylene, and chlorinated aromatic compounds such as chlorophenols and chlorobenzenes are formed during incineration. When flyash coexists in these precursors, the precursors change to dioxides under the catalytic action of flyash, and thus generated dioxins exist in incineration ashes.
Conventionally proposed methods for treating such dioxides-containing flyash are as follows. (1) To keep the dioxides-containing flash for 1-2 hours at a temperature between 320 and 400.degree. C. under a reductive atmosphere (for instance, 2 hours at 320.degree. C. or 1-1.5 hours at 340.degree. C.) (Hagenmaier process, "ORGANOHALOGEN COMPOUNDS Vol. 27 (1996)" p. 147-152)
(2) A heat treatment of the dioxins-containing flash at 300-500.degree. C. under the existence of a dioxins formation inhibitor (JPA 4-241880). It has been said that dioxins do not heat-decompose at a temperature lower than 300.degree. C. This method is basically in accordance with the above mentioned theory, decomposing dioxides at above 300.degree. C., and a dioxins formation inhibitor is added to the flash in order to prevent the formation of dioxins during the heat treatment in a temperature range in which dioxides decompose. For particulars, flyash is heated at 400.degree. C. for 2 hours under the existence of pyridine vapor as the dioxins formation inhibitor.
The aforementioned conventional methods have a drawback in that their high treatment temperature and long treatment time require much energy and high cost. Especially, in the above mentioned method (1) it is required to perform the treatment under a reductive atmosphere such as nitrogen gas, bringing about complexity and a high cost.